In recent years, with the increase of the number of pixels in imaging devices used in imaging apparatuses such as a digital still camera and a digital camcorder, an image data amount processed by integrated circuits embedded in such imaging apparatuses has also been increased. In order to process a large amount of data, a faster operation frequency, a larger memory capacity, and the like are considered to assure a bus width for data transfer in such an integrated circuit. However, they result directly in cost increase of the imaging apparatuses.
Generally, imaging apparatuses such as a digital still camera and a digital camcorder compress data, for which all image processes have been performed in integrated circuits, to be recorded onto external recording apparatuses such as a Secure Digital (SD) card. Therefore, the external recording apparatus having the same capacity can store image data having a larger size than non-compressed one or image data having more pictures. The compression is performed by using coding methods such as Joint Photographic Experts Group (JPEG) or Moving Picture Experts Group (MPEG).
Patent Literature 1 discloses that image data compression is performed also on pixel signals (raw data) provided from an imaging device. As a result, even if the advance of pixel increase in imaging devices increases load on signal processing, this technique can achieve high-speed operations by decreasing a bus bandwidth required for memory writing/reading. In addition, the technique adopts the fixed length coding method to assure a bus bandwidth and reduce a compression processing amount. In the method, a maximum value and a minimum value are calculated from pixel data in a given image region to determine a local dynamic range in the region, and then a value calculated by subtracting the minimum value from all pixels in the region is quantized with a quantization width according to the dynamic range. As a result, fixed length coding is achieved.
However, the imaging apparatus disclosed in Patent Literature 1 quantizes a difference value between a pixel and a minimum value in the same region, so that as the dynamic range in the region is increased, the quantization width is increased. This technique utilizes visual characteristics that as a dynamic range in a region is larger, the image is likely to have a large change so that a subtle level change is unlikely to be perceived. However, in the calculation of a minimum value and a maximum value in a region, there is a problem that compression cannot start until all pixels in the region are obtained. Therefore, although a bus bandwidth can be decreased, processing is delayed until pixels in the region are obtained. As a result, in the structure, another memory is necessary to store the pixels in the region, and thereby a circuit size is increased.
Patent Literature 2 discloses that a fixed length coding unit divides image data into unit blocks each having a small region regarding input pixel signals (raw data) received from an imaging device, then calculates an average value level of the image data in each unit block, and performs fixed length coding on the average value level, and an exposure control unit calculates a luminance level of the entire image data in each unit block based on the average value level and controls an exposure amount of an imaging apparatus so that the image data has a predetermined luminance level.